Network Working Group S. Valdez
Internet-Draft Google LLC
Intended status: Informational 13 July 2020
Expires: 14 January 2021
Privacy Pass: HTTP API
draft-svaldez-pp-http-api-01
Abstract
This document specifies an integration for Privacy Pass over an HTTP
API, along with recommendations on how key commitments are stored and
accessed by HTTP-based consumers.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Layout . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3. Requirements . . . . . . . . . . . . . . . . . . . . . . 3
2. Privacy Pass HTTP API Wrapping . . . . . . . . . . . . . . . 3
3. Server key registry . . . . . . . . . . . . . . . . . . . . . 3
3.1. Key Registry . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Server Configuration Retrieval . . . . . . . . . . . . . 5
4. Key Commitment Retrieval . . . . . . . . . . . . . . . . . . 5
5. Privacy Pass Issuance . . . . . . . . . . . . . . . . . . . . 7
6. Privacy Pass Redemption . . . . . . . . . . . . . . . . . . . 8
6.1. Generic Token Redemption . . . . . . . . . . . . . . . . 8
6.2. Direct Redemption . . . . . . . . . . . . . . . . . . . . 9
6.3. Delegated Redemption . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8.1. Well-Known URI . . . . . . . . . . . . . . . . . . . . . 11
9. Normative References . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The Privacy Pass protocol as described in
[draft-davidson-pp-protocol] can be integrated with a number of
different settings, from server to server communication to browsing
the internet.
In this document, we will provide an API to use for integrating
Privacy Pass with an HTTP framework. Providing the format of HTTP
requests and responses needed to implement the Privacy Pass protocol.
1.1. Terminology
We use the same definition of server and client that is used in
[draft-davidson-pp-protocol] and [draft-davidson-pp-architecture].
We assume that all protocol messages are encoded into raw byte format
before being sent. We use the TLS presentation language [RFC8446] to
describe the structure of protocol messages.
1.2. Layout
* Section 2: Describes the wrapping of messages within HTTP
requests/responses.
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* Section 3: Describes how HTTP clients retrieve server
configurations and key commitments.
* Section 5: Describes how issuance requests are performed via a
HTTP API.
* Section 6: Describes how redemption requests are performed via a
HTTP API.
1.3. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Privacy Pass HTTP API Wrapping
Messages from HTTP-based clients to HTTP-based servers are performed
as GET and POST requests. The messages are sent via the "Sec-
Privacy-Pass" header.
"Sec-Privacy-Pass" is a Dictionary Structured Header
[draft-ietf-httpbis-header-structure-15]. The dictionary has two
keys:
* "type" whose value is a String conveying the function that is
being performed with this request.
* "body" whose value is a byte sequence containing a Privacy Pass
protocol message.
Note that the requests may contain addition Headers, request data and
URL parameters that are not specified here, these extra fields should
be ignored, though may be used by the server to determine whether to
fulfill the requested issuance/redemption.
3. Server key registry
A client SHOULD fetch a server's current public key information prior
to performing issuance and redemption. This configuration is
accessible via a "CONFIG_ENDPOINT", either provided by the server or
by a global registry that provides consistency and anonymization
guarantees.
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3.1. Key Registry
To ensure that a server isn't providing different views of their
public key material to different users, servers are expected to write
their commitments to a verifiable data structure.
Using a verifiable log-backed map ([verifiable-data-structures]), the
server can publish their commitments to the log in a way that clients
can detect when the server is attempting to provide a split-view of
their key commitments to different clients.
The key to the map is the "server_origin", with the value being:
struct {
opaque public_key<1..2^16-1>;
uint64 expiry;
uint8 supported_methods; # 3:Issue/Redeem, 2:Redeem, 1:Issue
opaque signature<1..2^16-1>;
} KeyCommitment;
struct {
opaque server_id<1..2^16-1>;
uint16 ciphersuite;
opaque verification_key<1..2^16-1>;
KeyCommitment commitments<1..2^16-1>;
}
The addition to the log is made via a signed message to the log
operator, which verifies the authenticity against a public key
associated with that server origin (either via the Web PKI or a out-
of-band key). The signature should be computed under a long-term
signing key that is associated with the server identity.
The server SHOULD then store an inclusion proof of the current key
commitment so that it can present it when delivering the key
commitment directly to the client or when the key commitment is being
delivered by a delegated party (other registries/preloaded
configuration lists/etc).
The client can then perform a request for the key commitment against
either the global registry or the server as described in Section 4.
Note that the signature should be verified by the client to ensure
that the key material is owned by the server. This requires that the
client know the public verification key that is associated with the
server.
To avoid user segregation as a result of server configuration/
commitment rotation, the log operator SHOULD enforce limits on how
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many active commitments exist and how quickly the commitments are
being rotated. Clients SHOULD reject configurations/commitments that
violate their requirements for avoiding user segregation. These
considerations are discussed as part of
[draft-davidson-pp-architecture].
3.2. Server Configuration Retrieval
Inputs: - "server_origin": The origin to retrieve a server
configuration for.
No outputs.
1. The client makes an anonymous GET request to
"CONFIG_ENDPOINT"/.well-known/privacy-pass with a message of type
"fetch-config" and a body of:
struct {
opaque server_origin<1..2^16-1>;
}
1. The server looks up the configuration associated with the origin
"server_origin" and responds with a message of type "config" and
a body of:
struct {
opaque server_id<1..2^16-1>;
uint16 ciphersuite;
opaque commitment_id<1..2^8-1>;
opaque verification_key<1..2^16-1>;
}
1. The client then stores the associated configuration state under
the corresponding "server_origin".
(TODO: This might be mergable with key commitment retrieval if
server_id = server_origin)
4. Key Commitment Retrieval
The client SHOULD retrieve server key commitments prior to both an
issuance and redemption to verify the consistency of the keys and to
monitor for key rotation between issuance and redemption events.
Inputs: - "server_origin": The origin to retrieve a key commitment
for.
No outputs.
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1. The client fetches the configuration state "server_id",
"ciphersuite", "commitment_id" associated with "server_origin".
2. The client makes an anonymous GET request to
"CONFIG_ENDPOINT"/.well-known/privacy-pass with a message of type
"fetch-commitment" and a body of:
struct {
opaque server_id<1..2^16-1> = server_id;
opaque commitment_id<1..2^8-1> = commitment_id;
}
1. The server looks up the current configuration, and constructs a
list of commitments to return, noting whether a key commitment is
valid for issuance or redemption or both.
2. The server then responds with a message of type "commitment" and
a body of:
struct {
opaque public_key<1..2^16-1>;
uint64 expiry;
uint8 supported_methods; # 3:Issue/Redeem, 2:Redeem, 1:Issue
opaque signature<1..2^16-1>;
} KeyCommitment;
struct {
opaque server_id<1..2^16-1>;
uint16 ciphersuite;
opaque verification_key<1..2^16-1>;
KeyCommitment commitments<1..2^16-1>;
opaque inclusion_proofs<1..2^16-1>;
}
1. The client then verifies the signature for each key commitment
and stores the list of commitments to the current scope. The
client SHOULD NOT cache the commitments beyond the current scope,
as new commitments should be fetched for each independent
issuance and redemption request. The client SHOULD verify the
"inclusion_proofs" to confirm that the key commitment has been
submitted to a trusted registry. Once the client receives the
"ciphersuite" for the server, it should implement all Privacy
Pass API functions (as detailed in [draft-davidson-pp-protocol])
using this ciphersuite.
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5. Privacy Pass Issuance
Inputs: - "server_origin": The origin to request token issuance from.
- "count": The number of tokens to request issuance for.
Outputs: - "tokens": A list of tokens that have been signed via the
Privacy Pass protocol.
1. When a client wants to request tokens from a server, it should
first fetch a key commitment from the server via the process
described in Section 4 and keep the result as "commitment".
2. The client should then call the "Generate" function requesting
"count" tokens storing the resulting "input" data.
3. The client then makes a POST request to <"server_origin">/.well-
known/privacy-pass with a message of type "request-issuance" and
a body of:
enum { Normal(0) } IssuanceType;
struct {
IssuanceType type = 0;
opaque msg<0..2^16-1> = input.msg;
}
1. The server, upon receipt of the "request" should call the "Issue"
function with the "public_key", "secret_key" and the value of
"msg" with a result of "resp".
2. The server should then respond to the POST request with a message
of type "issue" and a body of:
struct {
IssuanceType type = request.type;
IssuanceResp resp = resp;
}
1. The client should then should call the "Process" function with
the "public_key", stored "inputs" and resulting "resp", to
extract a list of "redemption_tokens".
2. The client should store the "public_key" associated with these
tokens and the elements of "redemption_tokens" under storage
partitioned by the "server_origin", accessible only via the
Privacy Pass API.
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6. Privacy Pass Redemption
There are two forms of Privacy Pass redemption that could function
under the HTTP API. Either passing along a token directly to the
target endpoint, which would perform its own redemption Section 6.1,
or the client redeeming the token and passing the result along to the
target endpoint. These two methods are described below.
6.1. Generic Token Redemption
Inputs: - "server_id": The server ID to redeem a token against. -
"ciphersuite": The ciphersuite for this token. - "public_key": The
public key associated with this token. - "redemption_token": A
Privacy Pass token. - "info": Additional data to bind to this token
redemption.
Outputs: - "result": The result of the redemption from the server.
1. The client should call the "Redeem" function with
"redemption_token" and additional data of "info" storing the
resulting "data" and "tag".
2. The client makes a POST request to <"server_origin">/.well-known/
privacy-pass with a message of type "token-redemption" and a body
of:
struct {
opaque server_id<1..2^16-1> = server_id;
opaque data<1..2^16-1> = data;
opaque tag<1..2^16-1> = tag;
opaque info<1..2^16-1> = info;
}
1. The server, upon receipt of "request" should call the "Verify"
interface with "public_key", "secret_key" and the received
"data", "tag", "info" storing the resulting "resp".
2. The server should then respond to the POST request with a message
of type "redemption-result" and a signed body of:
struct {
opaque info<1..2^16-1> = info;
uint8 result = resp;
// signature of info and result using
// the server's verification key.
opaque signature<1..2^16-1>;
}
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1. The client upon receipt of this message should verify the
"signature" using the "verification_key" from the configuration
and return the "result".
6.2. Direct Redemption
Inputs: - "server_origin": The server origin to redeem a token for. -
"target": The target endpoint to send the token to. -
"additional_data": Additional data to bind to this redemption
request.
1. When a client wants to redeem tokens for a server, it should
first fetch a key commitment from the server via the process
described in Section 4 and keep the result as "commitment".
2. The client should then look up the storage partition associated
with "server_origin" and fetch a "redemption_token" and
"public_key".
3. The client should verify that the "public_key" is in the current
"commitment". If not, it should discard the token and fail the
redemption attempt.
4. As part of the request to "target", the client will include the
token as part of the request in the "Sec-Privacy-Pass" header
along with whatever other parameters are being passed as part of
the request to "target". The header will contain a message of
type "token-redemption" with a body of:
struct {
opaque server_id<1..2^16-1> = server_id;
uint16 ciphersuite = ciphersuite;
opaque public_key<1..2^16-1> = public_key;
RedemptionToken token<1..2^16-1> = redemption_token;
opaque additional_data<1..2^16-1> = additional_data;
}
At this point, the "target" can perform a generic redemption as
described in Section 6.1 by forwarding the message included in the
request to "target".
6.3. Delegated Redemption
Inputs: - "server_origin": The server origin to redeem a token for. -
"target": The target endpoint to send the token to. -
"additional_data": Additional data to bind to this redemption
request.
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1. When a client wants to redeem tokens for a server, it should
first fetch a key commitment from the server via the process
described in Section 4 and keep the result as "commitment".
2. The client should then look up the storage partition associated
with "server_origin" and fetch a "redemption_token" and
"public_key".
3. The client should verify that the "public_key" is in the current
"commitment". If not, it should discard the token and fail the
redemption attempt.
4. The client constructs a bytestring "info" made up of the
"target", the current "timestamp", and "additional_data":
struct {
opaque target<1..2^16-1>;
uint64 timestamp;
opaque additional_data<0..2^16-1>;
}
1. The client then performs a token redemption as described in
Section 6.1. Storing the resulting "redemption-result" message.
2. As part of the request to "target", the client will include the
redemption result as part of the request in the "Sec-Privacy-
Pass" header along with whatever other parameters are being
passed as part of the request to "target". The header will
contain a message of type "signed-redemption-result" with a body
of:
struct {
opaque server_origin<1..2^16-1>;
opaque target<1..2^16-1>;
uint64 timestamp;
opaque additional_data<1..2^16-1> = additional_data;
opaque signed_redemption<1..2^16-1>;
}
At this point, the "target" can verify the integrity of
"signed_redemption.info" based on the values of "target",
"timestamp", and "additional_data" and verify the signature of the
redemption result by querying the current configuration of the
Privacy Pass server. The inclusion of "target" and "timestamp"
proves that the server attested to the validity of the token in
relation to this particular request.
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7. Security Considerations
Security considerations for Privacy Pass are discussed in
[draft-davidson-pp-architecture].
8. IANA Considerations
8.1. Well-Known URI
This specification registers a new well-known URI.
URI suffix: "privacy-pass"
Change controller: IETF.
Specification document(s): this specification
9. Normative References
[draft-davidson-pp-architecture]
Davidson, A., "Privacy Pass: Architectural Framework",
n.d., <https://tools.ietf.org/html/draft-davidson-pp-
architecture-00>.
[draft-davidson-pp-protocol]
Davidson, A., "Privacy Pass: The Protocol", n.d.,
<https://tools.ietf.org/html/draft-davidson-pp-protocol-
00>.
[draft-ietf-httpbis-header-structure-15]
Nottingham, M. and P-H. Kamp, "Structured Headers for
HTTP", n.d., <https://tools.ietf.org/html/draft-ietf-
httpbis-header-structure-15>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[verifiable-data-structures]
"Verifiable Data Structures", n.d.,
<https://github.com/google/trillian/blob/master/docs/
papers/VerifiableDataStructures.pdf>.
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Author's Address
Steven Valdez
Google LLC
Email: svaldez@chromium.org
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